Why not? Who hasn’t dreamed about casting off the bonds of gravity and soaring through the air like Superman or sticking to the side of buildings like Spider-Man?

Unfortunately, physics has a way of ruining a good fantasy. After all, superheroes only exist in comic books and movies. People can't really fly, shoot laser beams or bench-press pickup trucks.

Or can they?

Traditionally, heroes develop their powers in unusual ways — radiation exposure, mutant genes, even deals with the devil. As a result, they can lift cars, control the weather or change their appearance. Is any of this even remotely plausible? Do any of the comic-book methods for gaining superpowers actually work?

Superman was lucky enough to gain his powers as an accident of birth. Born Kal-El, the sole survivor of the destroyed planet Krypton, his alien anatomy is supercharged by the light of our yellow sun, giving him abilities as varied as flight, heat vision and bullet-proof skin.

Much of Superman's story doesn't ring true, says Phil Plait, an astronomer who runs Bad Astronomy, a Web site dedicated to correcting misconceptions about our universe and promoting good science. "I can't think of any way of making a planet explode," he says. "It would take a huge amount of energy."

But there could be a kernel of truth to the Superman legend. "Krypton orbited a star that was a red giant," says Plait. "The difference between a red giant and our sun is that a red giant is cooler. There are other things, but a star like the sun is hotter, so it puts out more light in the ultraviolet, in the blue part of the spectrum. So if you could think of some way that ultraviolet light would take someone like Kal-El and make him able to fly around, then more power to you."

Of course, those of us born here on Earth have to find other ways to fuel our superpowers. Spider-Man, The Incredible Hulk and Daredevil all benefited from exposure to radioactive materials — what about giving yourself a dose of some unstable atomic nuclei to jump-start your powers?

Not a good idea, according to radiation safety consultant George J. Vargo. "The bottom line is, what all of this stuff does when it enters the body is collide with atoms and molecules," he says. Small doses of radiation cause damage that can easily be repaired. A little more can make you very sick or result in the loss of limbs through amputation. Crank up the dosage past 600 rem (a standard unit for measuring radiation) — an amount 1,600 times what the average person is exposed to every year — and you'll be dead within 14 days. "That's a one-way trip," says Vargo. "You damage the cells that line the small intestine, begin to lose one of the body's most important fluid barriers, and quite literally you leak out the gut."

Given enough money, you could probably buy your way into the crime-fighting game. Batman doesn't have any superpowers — just a superwallet. The Caped Crusader's alter ego, Bruce Wayne, is the owner of Wayne Enterprises, a massive international technology conglomerate. In our Forbes Fictional Fifteen listing of the richest fictional characters, we estimated his net worth at $6.3 billion. If Wayne was a real guy, he'd be the 33rd richest person in America, tied with Fidelity Investments' Edward Crosby Johnson III, and right behind News Corp.'s Rupert Murdoch.

Bruce Wayne uses his riches to equip himself with high-powered military hardware. It's a recipe that could certainly work in the real world, but at a hefty price: In our story " Being Batman," we estimated Batman's startup costs at over $3.3 million.

So what to do if you're not a billionaire or an extraterrestrial? The good folks at DC and Marvel comics have come up with lots of ways to turn man into superman. Join us as we examine whether any of the comic book super power-ups would actually work.